This invention relates to medical tube assemblies.
Tracheostomy tubes are inserted through a surgically-made opening in the throat so that one end of the tube locates in the trachea and the other end projects externally. Ventilation gas can then be supplied to the patient's airways via the tube or the patient can breath normally through the tube.
Tracheostomy tubes are often used where prolonged ventilation is required. In use, however, the inside of the tube becomes coated with secretions, which can reduce the flow of gas along the tube and provide a site for bacteria to multiply. Periodic removal and replacement of the tube is relatively traumatic and uncomfortable for the patient, and must be carried out by surgical staff. It is, therefore, preferable to clean the tube, either by using a suction catheter inserted along the bore of the tube or, preferably, by using an inner cannula. Where an inner cannula is used, this is configured to form a close sliding fit within the tube extending along its entire length. The inner cannula remains in place during normal ventilation and, when secretions have built up, it is removed and replaced by a new inner cannula. This replacement procedure can be carried out regularly by nursing staff so provides an efficient and effective arrangement for maintaining effective ventilation and reducing the risk of respiratory infection. Examples of tracheostomy tube assemblies with an outer tube and an inner cannula are described in, for example, GB2056285, U.S. Pat. Nos. 4,315,505, 4,817,598, 5,119,811, 5,184,611, 5,386,826, 6,019,753, 6,135,110, GBO800112.5, WO94/01156 and WO04/101048.
There are, however, problems with inner cannulae. The cannulae do reduce the cross-sectional area of the effective passage through the tube so it is important for the wall thickness to be as small as possible and for the cannula to be a close sliding fit within the outer tube whilst allowing free insertion and removal. It is also important that the inner cannula does not buckle, or otherwise deform in a manner that would reduce gas flow, when inserted along the outer tube, which may not be smoothly curved. The inner cannula should extend along the entire length of the outer tube without projecting from its end and should preferably form an effective seal at the patient end to prevent seepage of material between the inner wall of the inner cannula and the outer wall of the inner cannula. It has been found that a most effective assembly can be provided if the patient end of the outer tube is formed with a shallow inturned lip, in the manner described in U.S. Pat. No. 4,315,515. The inner cannula is configured so that its patient end abuts the rear-facing surface of this lip. In this way, the lip prevents the inner cannula projecting beyond the outer tube and also helps improve the seal between the patient end of the inner cannula and outer tube. One problem, however, with this arrangement is that when the inner cannula has been removed and a suction catheter or other elongate device is inserted through the outer tube to extend beyond the patient end of the outer tube, there is a tendency for the tip of the catheter or device to catch on the inturned lip at the patient end of the tube.